1. Field of the Invention
This invention relates to a multilayer circuit board having a structure in which a plurality of resin film layers each formed with an interlayer connection wiring and a surface wiring are laminated, and to a method of fabricating the multilayer circuit board.
2. Description of Related Art
Recently, with the increase in the integration density of semiconductor integrated circuits, multilayer circuit boards for forming high-density integrated circuits thereon with finer circuit patterns have been developed. With respect to such multilayer circuit boards, high circuit connection reliability is required while there is a strong demand for a reduction in the period of order-delivery time.
To form finer circuit patterns, a multilayer circuit board comprising a plurality of resin films is used. Via holes are formed in each of the resin films by using a lithography technique, an interlayer connection wiring element is formed in each via hole, and a surface wiring element connected to the interlayer connection wiring element is formed on a surface of each resin film. This process is repeated to obtain a multilayer circuit board. This conventional process for fabricating a multilayer circuit board will be described in more detail.
Referring to FIG. 1A, (in step (a)) surface wiring elements 101.sub.1 are formed on a surface of a resin film 100.sub.1. The surface wiring elements 101.sub.1 are formed by etching a foil attached to the surface of the resin film 100.sub.1 or a metal film deposited by a well-known thin film technique such as sputtering. Next, (in step (b)) a precursor of polyimide, for example, is applied to the surface of the resin film 100.sub.1 and is cured to form a layer-insulation film 100.sub.2 having a thickness of 10 to 20 .mu.m. Thereafter, (in step (c)) via holes 102 having a diameter of 20 to 50 .mu.m are formed in the layer-insulation film 100.sub.2 at predetermined positions. A portion of each of the surface wiring elements 101.sub.1 is exposed in corresponding via holes 102. Ordinarily, the via holes 102 are formed by, for example, a well-known lithography technique.
Thereafter (in step (d)), as shown in FIG. 1B, interlayer connection wiring elements 103 made of, for example, copper (Cu) are formed on inner walls of the via holes 102. The interlayer connection wiring elements 103 are connected to the surface wiring elements 101.sub.1 exposed in the via holes 102. Subsequently, (in step (e)) surface wiring elements 101.sub.2 are formed of copper (Cu) on the surface of the layer-insulation film 100.sub.2. The surface wiring elements 101.sub.2 are connected to the interlayer connection wiring elements 103. The surface wiring elements 101.sub.1 and 101.sub.2 are connected to each other through the interlayer connection wiring elements 103. If the surface wiring elements 101.sub.2 are comparatively thick, the internal spaces of the via holes 102 are filled with the material of the surface wiring elements 101.sub.2. The interlayer connection wiring elements 103 and the surface wiring elements 101.sub.2 may be formed by a well-known thin film technique such as vacuum deposition or sputtering and a well-known lithography technique including a lift-off technique.
The steps (b) to (e) are repeated to fabricate a multilayer circuit board having a desired number of layers.
The above-described conventional method of fabricating a multilayer circuit board formed of resin films requires many fabrication days. To make one layer of circuit base by the above-described steps (b) to (e), three days, on average, are required. That is, fifteen days are required to complete a multilayer circuit board formed of five layers of circuit bases. In such a case, it is possible that an urgent demand for a design change with respect to a portion of the multilayer circuit board cannot be satisfied. There is, therefore, a need to reduce the time taken to fabricate the above-described multilayer circuit board formed of resin films.